PASADENA, Calif. — Since he's always asking people about their lives in his role as a CNN reporter, Anderson Cooper said it felt only natural that he do the same with his own life and that of his famous mother, Gloria Vanderbilt.

He also believes the upcoming HBO film, "Nothing Left Unsaid: Gloria Vanderbilt & Anderson Cooper," can be a lesson to viewers not to miss an opportunity to learn as much as possible about their parents when they still have the opportunity.

There was much for Cooper to find out. His mother, born into one of the richest families in America and now 91, was the subject of a high-profile custody battle when she was a child, dated stars like Frank Sinatra, famously launched a brand of designer jeans under her name and watched her son — Cooper's older brother — commit suicide.

"My mom has been in the public eye probably longer than anybody else alive," Cooper said on Thursday. The film debuts April 9 on HBO.

He recalls watching an old movie once with her mother and asking if she knew the star, Errol Flynn. "She said, 'Oh, yes,' " and Cooper learned that his mom once dated Flynn when she was 17.

Cooper, 48, was the instigator of the project. He has been spending a lot of time over the past decade going through boxes of memorabilia, filled with art and letters, in her home. In one, he found a box of love letters from Howard Hughes.

"That sounds like that show 'The Hoarders,' " Vanderbilt interjected at a meeting with reporters. "It wasn't really quite like that."

Cooper interviews his mother throughout the documentary, and he gives to filmmaker Liz Garbus his own tapes of conversations with his mother though the years. As an outsider, Garbus was able to make connections and observations that he would never have thought of, Cooper said.

"I know my mom in a way I didn't know her a year and a half ago when we started this process," he said.

There is plenty of sadness in their lives, particularly the death of Cooper's father at age 50 and Carter Cooper's suicide. In a lengthy scene at the end of the movie, Cooper and his mother visit Carter's grave on a winter day.

Despite the evident pain and her regrets, Vanderbilt said talking about Carter is a way to keep him alive to her. "If one is telling the truth, there is nothing to fear and it is very freeing," she said.